This invention relates to an method for injection molding of synthetic resinous materials, and more particularly to an improved injection molding screw.
The injection molding of synthetic resinous materials has long been known and a wide variety of devices have been used for plasticating material prior to its injection into a mold. Screws are particularly well suited for this purpose.
In order to obtain acceptable injection molded products, the plasticated synthetic material must have homogeneous properties. More particularly, the temperature distribution throughout the material must be highly uniform in order to avoid generating internal stresses in the molded product. In addition, the plasticated material must be well mixed in those instances where additives such as colorants or reinforcing materials are mixed with the plasticated material prior to injection. Lack of adequate mixing where additives are used can affect not only the surface appearance of a molded article but also the structural integrity of the product.
Another difficulty to be avoided is the presence of incompletely plasticated particles of the particulate feed material in the accumulating reservoir of plasticated material in the machine. Such particles are detrimental and can obstruct the internal passages of a mold causing unacceptable products to be molded.
Another complicating factor in the design of injection molding machines is the fact that the barrel and the rotatable reciprocable screw are cantilever-mounted. Accordingly, the processing portions of the screw cannot be increased in length to improve mixing and temperature distribution without regard to the effect that such a length increase would have on the overall design of the machine. Moreover, since the screw ordinarily reciprocates through a distance of approximately four times the screw diameter, a comparatively long length is required for the feed section of the screw in which the particulate material is initially received and compacted. The length of the feed section thus imposes a demand for high efficiency in downstream processing sections.
To improve plasticated material uniformity some injection molding machine includes a screw provided with a mixing section in which the plasticated material is forced to mix with itself. The addition of a mixing section to an injection molding machine screw necessarily increases the length of that screw, the length of the surrounding barrel, the cost of the machine, and requires adjustment of the length and position of other processing section of the machine screw. Also, if the mixing section design includes close-clearance barriers in primary flow channels, this can easily trap tramp metal, causing rapid wear.
Accordingly, it will be seen that the need continues to exist for an injection molding machine have more efficient processing sections to produce a homogeneous reservoir of plasticated synthetic resinous material for injection molding purposes. The present invention enables improved melting capacity for the screw in a relatively short length barrel, which is very desirable in injection molding machines.